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Extending LANs Fiber modems Repeaters Bridges Switches Distance limitation in LANs MAC protocols such as CSMA/CD require time proportional to the length of the cable Electrical signal weakens with distance

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Extending LANs

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Extending lans l.jpg

Extending LANs

  • Fiber modems

  • Repeaters

  • Bridges

  • Switches


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Distance limitation in LANs

  • MAC protocols such as CSMA/CD require time proportional to the length of the cable

  • Electrical signal weakens with distance

  • Network designers choose a combination of capacity, delay and distance that can be achieved within a given cost


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Fiber optic extensions

  • Fiber modems extend connection between computer and transceiver


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Repeaters

  • Join Ethernet cables (segments) together

  • Amplified signal - no knowledge of frames

  • Deals with signal strength, but not delay


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Multiple repeaters

  • Ethernet standard says no more than four repeaters between two computers


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  • Fiber modems can be used between repeaters for long distance extensions

  • Biggest problem with repeaters is that they transmit all signals including collisions and noise which limits scalability


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Bridges

  • Connect two segments, but work at the frame level

  • Use promiscuous mode and forward all frames

  • Don’t forward erroneous frames (e.g., collisions and noise)


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Frame filtering

  • Only forward a frame if necessary

    • destination is on the other segment

    • broadcast address is used

  • Bridge learns which segment a computer is on when that computer sends a frame

  • When a frame arrives the bridge

    • extracts source address and updates knowledge

    • inspects destination address for forwarding


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Example of learning in a bridge


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Planning a bridged network

  • Propagation principle - a bridge will only forward frames as far as is necessary

  • Bridges allow communication on separate segments to occur at the same time

  • Plan the network so that computers that communicate frequently are on the same segment

  • May be possible to improve an existing LAN’s performance by adding a bridge


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Bridging between buildings


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Bridging longer distances


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A cycle of bridges

  • Compute a distributed spanning tree


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Switching

  • Switch - a single electronic device that transfers frames between computers

  • Whereas a hub simulates a shared medium, a switch simulates a bridged LAN with one computer per segment

  • Advantage is greater data rate due to parallelism

  • Some organisations combine hubs and switches to reduce cost


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The concept of a switched LAN


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Summary

  • Fiber optic extensions

  • Repeaters and multiple repeaters

  • Bridges

    • frame filtering

    • bridging distances

  • Switches


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Summary of LANs

  • Locality of reference

  • LAN topologies

  • MAC protocols, especially CSMA/CD

  • Hardware addressing and frame types

  • LAN wiring

  • Extending LANs


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Exam question

  • http://www.cs.nott.ac.uk/~mvr/2-CCN.doc


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